An apparatus that emits pulsed light, receives light reflected or scattered from an object located in a preset measuring range by means of a photosensor having a plurality of pixels, and generates range images from the output from the photosensor is known. (for example, refer to Published Japanese translation No. 2002-500367 of PCT application).
A range image represents the distance to a photographed object by image gradations (pixel values). The use of a range image makes it possible to measure at a time the distances to a plurality of objects located in a wide range.
In the apparatus disclosed in Published Japanese Application No. 2002-500367, as shown in FIG. 12A, the pulse width of the pulsed light and the exposure period (exposure time) of the photosensor are set to ΔL and ΔA (≦ΔL), respectively, and the emission of the pulsed light and the exposure of the photosensor start simultaneously so that each pixel of the photosensor stores a charge according to the distance D to the object.
In other words, the reception of the light reflected or scattered from the object is delayed from the emission of the pulsed light by time τ(=2D/VC, where VC is the speed of light) according to the distance R to the object. For this reason, each pixel of the photosensor receives the reflected or scattered light for the period (charge storage time) ΔA−τ during the exposure period. That is, the longer the distance R to the object is, the less the charge Q stored in each pixel of the photosensor is.
However, even if the storage time is the same, the stored charge Q increases as the intensity of the reflected or scattered light increases. Also, even if the distance to the object is the same, the intensity of the reflected or scattered light varies with the light reflection coefficient or light scattering coefficient of the surface of the object. For this reason, it is necessary to standardize the stored charge Q in order to obtain from the charge a pixel value of the range image correctly representing the distance to the object.
The apparatus disclosed in Published Japanese Application No. 2002-500367 carries out this standardization by setting the exposure time ΔB of the photosensor so as to enable definite reception of all the reflected or scattered light and measure the charge Qc per unit time (pulse width ΔL) due to the reflected or scattered light from the object separately from the stored charge Q related to the distance R.
By finding the ratio between the stored charge Qc per unit time and the stored charge Q related to the distance R obtained by the previous measurement(=Q/Qc), a pixel value can be obtained that is standardized so as to be related only to the distance R to the object and unrelated to the intensity of the reflected or scattered light.
The pulsed light emitted toward the measuring range not only scatters while it is flying, but is also absorbed and scattered by the surface of the object. For this reason, the intensity of the reflected or scattered light received by the photosensor, and consequently the output from the photosensor, is extremely low. It is therefore necessary to increase the emission intensity of the pulsed light to enable detection of objects positioned at points distant from the apparatus and having a low reflection coefficient (for example, a black object).
However, the output power of a light emitting element such as a light emitting diode or a laser diode employed as a pulsed light source is difficult to boost without sacrificing high speed due to such constraints as the effect of parasitic elements and the current capacity of the power MOSFET that drives the light emitting element. Actually, the rise time of the pulsed light emitted by a light emitting element for emission of several watts is on the order of several hundred nanoseconds. Particularly, if the driver of the light emitting element includes a CMOS inverter, the rise time of the pulsed light, when charges are supplied via a PMOSFET, is even longer because the current driving capacity of a PMOSFET is low compared to an NMOSFET.
If the object is in relatively close range, for example, within 10 meters, the delay time τ (light flight time) of the reflected or scattered light with respect to the emitted light is 67 nanoseconds or less, which is obviously shorter than the rise time of the pulsed light. That is, as shown in FIG. 12B, the conventional apparatus has the problem in that a measurement of good accuracy cannot be carried out because measurement is performed using the rise period, when the light intensity varies.
When obtaining the stored charge Qc per unit time in the conventional apparatus, because the exposure time ΔB is set longer than the pulse width ΔL of the pulsed light, the period for receiving only the background light, which becomes noise, during the exposure period is longer, so that the S/N ratio of the stored charge Qc is unnecessarily decreased. This leads as a result to the problem of further degrading the measurement accuracy.
In the conventional apparatus, when obtaining the stored charge Q related to the distance D, the charge storage time increases to as long as when receiving light that is reflected or scattered from close range having a strong reception intensity. Because of this, saturation of the output (stored charge) from the photosensor easily occurs. Lowering the emission intensity to prevent this saturation gives rise to the problem of restricting the measuring range (the maximum allowable distance for detection of the object).